1. Field of the Invention
The invention relates to the field of micromachined sensors and actuators and in particular to post-release enhancement of detection and actuation capacitances in micromachined sensors and actuators.
2. Description of the Prior Art
Capacitive detection and actuation are commonly used in micromachined devices due to simplicity of implementation and effectiveness. However, the performance of capacitive sensors and actuators highly depends on the nominal capacitance of the microsystem. For example, in capacitive micromachined inertial sensors (i.e. accelerometers, gyroscopes, etc.) the performance is generally defined by the nominal capacitance of the sensing electrodes. Furthermore, in electrostatically actuated devices, the nominal actuation capacitance determines the required drive voltages. For a small actuation capacitance, large voltages are needed to achieve sufficient forces, which in turn results in a large drive signal feed-through. Thus, it is desired to maximize the sensing capacitance, and minimize the actuation voltages by increasing the actuation capacitance. However, the sensing and actuation capacitances of micromachined devices are limited by the minimum-gap requirement of the fabrication process.
In the micro-domain, capacitive sensing and actuation offer several benefits when compared to other sensing and actuation means (piezoresistive, piezoelectric optical, magnetic, etc.) with their ease of fabrication and integration, good DC response and noise performance, high sensitivity, low drift, and low temperature sensitivity. However, the performance of micromachined sensors employing capacitive detection is generally determined by the nominal capacitance of the sensing electrodes. Even though increasing the overall sensing area provides improved sensing capacitance, the sensing electrode gap is the foremost factor that defines the upper bound. Various advanced fabrication technologies have been reported (e.g. oxidation machining) that provide minimal electrode gap.
However, all of these approaches require additional expensive fabrication steps. In electrostatically actuated devices such as micromachined gyroscopes, the nominal actuation capacitance determines the required drive voltages. For a small actuation capacitance, large voltages are needed to achieve sufficient forces, which in turn results in a large drive signal feed-through. The drive signal feed-through is generally a major noise source, and often a larger signal than the measured Coriolis signal. Thus, these devices are conventionally operated in vacuum to achieve large amplitudes with low actuation voltages to minimize the drive feed-through, which results in an extremely narrow response bandwidth. Similarly, the force generated by the electrostatic actuation electrodes (comb-drives or parallel-plates) is limited by the minimum gap attainable in the used fabrication process. MEMS designers are facing challenges similar to those exemplified above while implementing other electrostatic MEMS sensors and actuators.
The following section analytically illustrates the dependence of sensing and actuation capacitances on the design and fabrication parameters. Then, prior techniques to enhance capacitance are presented.